Daventry has an unusually clear split between its two kinds of loft, because so much of the town went up in a single burst from the 1960s onward.
That estate housing is almost entirely trussed. A trussed roof has no usable space until the structure is reworked, and reworking it is a proper structural job rather than a matter of boarding out and putting a window in. It is entirely doable and we do it often, but a trussed roof and a cut roof are not the same job and should never carry the same price. Anyone quoting one as the other has not been up there.
In the older core the roofs are cut, which usually makes for a better conversion, but head height needs measuring rather than assuming. A roof can look generous from the hatch and still leave you short of two metres along the walking line once the floor build up and insulation are in.
The permitted development volume allowance is forty cubic metres on a terrace and fifty on a semi or detached house. A dormer facing the road is not permitted development anywhere, and in the town centre conservation area, where the Article 4 direction already controls windows and porches, anything altering the street elevation will be looked at closely. Rear dormers and rooflights are much easier.
Escape, fire separation and stair position are Building Regulations matters. Our loft conversion page goes through the options.